Optical coherence tomography (“OCT”) techniques generally provides cross-sectional images of biological samples with a resolution on the scale of several to tens of microns. Conventional OCT techniques, such as time-domain OCT (“TD-OCT”) techniques, can generally use low-coherence interferometry procedures to achieve depth ranging within a sample. In contrast, Fourier-Domain OCT (“FD-OCT”) techniques can use spectral-radar procedures to achieve depth ranging within the sample. FD-OCT techniques allow higher imaging speeds dues to an improved signal-to-noise performance and an elimination of a mechanically-scanned interferometer reference arm. A standard implementation of the spectral ranging technique in the FD-OCT systems does not provide an ability to discriminate between objects at positive and negative displacements relative to the interferometric path-matched depth. This likely depth degeneracy (alternately referred to as complex conjugate ambiguity) may limit the imaging depth within the sample to either positive or negative depths (which may prevent depth ranging ambiguity), effectively reducing the inherent imaging depth by a predetermined factor (e.g., a factor of two).
Depth degeneracy in the FD-OCT systems can result from the detection of only the real component of the generally complex interference fringe between the sample arm and the reference arm. If the complex interferogram is detected, the above-described depth degeneracy can be eliminated or at least reduced. Various demodulation techniques have been implemented to allow for a measurement of the complex interferogram. Such conventional techniques include phase shifting techniques, fused 3×3 coupler demodulation techniques, and frequency-shifting techniques. The phase shifting techniques generally use an active phase modulator element in the interferometer to dynamically adjust the relative phase between the sample arm and the reference arm. Multiple interferograms at various phase shifts may be measured and combined to produce the complex interferogram. One of the disadvantages of this conventional technique is that the interferograms are measured successively in time. This type of measurement reduces the system imaging speed, and allows for phase-drifts in the interferometer to degrade the measurement accuracy. The fused 3×3 couplers can yield interferograms on each of the 3 output ports that are phase-shifted relative to one another. The phase shift may depend on the coupling ratio. For example, these outputs can be detected and recombined to yield the complex interferogram if the relative phase relationships are known. High temperature, wavelength, and polarization sensitivity of the fused 3×3 (and in general fused N×N) coupler is used in a limited manner in many interferometer demodulation schemes as requiring an accurate demodulation. Conventional frequency shifting techniques have been successfully applied to optical frequency domain imaging systems. However, these techniques are not know to have been used in the SD-OCT systems. One of the reasons therefor is that such frequency shifting techniques usually utilize active elements, and have potentially limited optical bandwidths. Further, these techniques are generally not directly compatible with nonlinear triggering to remove source sweep nonlinearities.
Accordingly, there is a need to overcome the deficiencies as described herein above.